Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Banks
Line of Credit Facility [Line Items]
Debt outstanding	$ 5,200
Debt Instrument, Covenant Compliance	The Company was in compliance with its debt covenants as of December 31, 2015. The Company was in compliance with its debt covenants as of December 31, 2015.
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit agreement term	5 years
Maximum borrowing capacity under credit facility	$ 1,500
Number of participating banks | Banks	16
Total commitment amount under option to increase borrowing capacity	$ 2,000
Expiration date	Dec. 31, 2019
Covenant terms	This agreement has certain covenants, including a financial covenant requiring the Company to maintain a leverage ratio of total consolidated debt-to-consolidated capitalization (each defined in the credit agreement) at or below 0.50.
Leverage ratio covenant	50.00%
Additional borrowing capacity under maximum debt coverage covenant	$ 7,900
Line of Credit Facility, Covenant Compliance	The Company was in compliance with its debt covenants as of December 31, 2015.
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	$ 500
Letters of credit outstanding	$ 19
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	3
Percent held by each primary commitment holder	12.00%
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	13
Collective percentage held by secondary commitment holders	64.00%